In the late 1960s the then-available data from several laboratories suggested that microbes exhibited a constant forward mutation rate of about 0.003 per genome per replication; genome sizes varied by about 1000-fold, and so, inversely, did mutation rates per base pair per replication. This observation suggested that mutation rates had evolved to an optimum that was surprisingly constant among diverse organisms. Since then, the published data base has improved for many organisms and the invariance of per-genome mutation rates appears not only still to be the norm, but to extend all the way from a bacteriophage containing single-stranded DNA to microbial eukaryotes. However, RNA viruses and metazoans appear to possess mutation rates quite different from those of DNA-based microbes.